We have investigated the expression of surface membrane binding sites for tetanus toxin and alpha-scorpion toxin (AaHII) on cells of the in vivo developing mouse nervous system. There is a close temporal correlation in the pattern of emergence and accumulation of tetanus toxin binding cells (TBC) and that of post-mitotic neurons. In different nervous system areas, the fluctuations in relative TBC abundance reflect regional changes in the dynamics of neuronal subpopulations. The results indicate that the acquisition of membrane tetanus toxin binding sites may represent one of the earliest detectable characteristics of nascent neurons. The Na+ channel-associated scorpion toxin become detectable in fetal mouse brain two days after the appearance of TBC. Their density increases with fetal age without change in receptor properties. At all stages, scorpion toxin binds to a single class of noninteracting sites with a KD = 0.1 - 0.5 nM. The affinity of binding is voltage-dependent. Studies on brain cells and various cell lines grown in vitro suggest a selective association of the high affinity scorpion toxin receptors with neuronal phenotype. In culture, as in vivo, there is a time dependent increase in receptor density. These results indicate that both tetanus toxin and scorpion toxin can be used as qualitative markers of neuronal differentiation; moreover, estimates of the density of scorpion toxin binding sites provide a quantitative index of neuronal maturation.